In our prior application Serial No. 186,764, a description is given of a process for the polymerization and copolymerization of .alpha.-olefins in which the operation is carried out in the presence of a catalytic system comprising an organometallic compound of a metal of Groups Ia, IIa, IIb, IIIa, and IVa of the Periodic Table and a cocatalyst obtained by reacting a halogenated alumina with a certain atomic ratio of halogen/ aluminum and a derivative of a metal of Groups IVb, Vb, and VIb of the Periodic Table.
These catalytic systems make it possible to obtain, with high productivities (of the order of 1500 g. of polymer per gram of catalyst component, for example) polymers which are characterized in particularly high average molecular weights and, therefore, particularly low fluidity indices. The polymers of this type are very suitable for certain particular uses such as the manufacture of large capacity containers. However, for many applications it is preferred to use polymers with lower mean molecular weights and, thus, higher fluidity indices.
The catalytic systems described in our prior application do not make it possible to obtain these polymers with lower molecular weight except by using very substantial relative amounts of modification agent for the molecular weight (hydrogen). But, in this case, we note a very substantial decrease of the productivity of these catalytic systems.